1. Field of the Invention
The present invention relates to a so-called grating projection type moire apparatus; and, in particular, to a moire apparatus equipped with a fringe scanning function.
2. Description of the Prior Art
Moire apparatus have conventionally been known as an apparatus for easily capturing three-dimensional form information of an object to be measured in a short time. As the moire apparatus, those of a grating irradiation type and a grating projection type have been known, among which the degree of freedom in measurement of the object to be measured is higher in the latter since they do not need such a large reference grating as that of the former.
The grating projection type moire apparatus comprises a projection optical system and an observation optical system having their respective optical axes parallel to each other, and is configured such that an image of a projection grating is projected by the projection optical system onto an object to be measured, a deformed grating image formed on the object is focused by the observation optical system onto an observation reference grating, and the resulting moire fringe is observed. Here, if fringe scanning, by which the projection grating is moved in a direction orthogonal to the grating lines of both gratings within a plane orthogonal to both optical axes, is carried out, then whether the object is protruded or depressed can be determined, and phase connection can be made, whereby three-dimensional form information of the object can be obtained.
Since the moire fringe appearing on the monitor for observing the moire fringe is captured by way of the observation optical system, the actual depth of the moire fringe formed at a position on the object to be measured near the taking lens of the observation optical system differs from that of the moire fringe formed at a position far from the taking lens, even when they have the same grating line interval. Consequently, for obtaining correct three-dimensional form information of the object to be measured, it is necessary to correct the image magnification and sensitivity according to the position of each point on the object in the depth direction.
While it is necessary to carry out the image magnification correction and sensitivity correction according to the absolute distance between each point of the object to be measured and the taking lens of the observation optical system, the distance between the object and the taking lens must be measured separately from the moire fringe observation, since the three-dimensional data of the object computed from the moire fringe is inevitably relative positional data on the object.
This distance measurement is mainly carried out by a manual operation in which a measure is placed between a given point on the object and the taking lens, and its scale is read out. In such a measuring method, it is difficult to correctly measure the distance. As a consequence, it is problematic in that the image magnification correction cannot be carried out accurately.
Also, while the distance measurement is carried out by the above-mentioned manual operation with a measure or the like, or measurement using a measuring device or the like, the point subjected to the distance measurement on the object to be measured is also a reference point for the image magnification correction and sensitivity correction. Consequently, for accurately performing the image magnification correction and sensitivity correction, it is important to correctly grasp the position of this point within a plane orthogonal to the depth direction.
Further, while the above-mentioned fringe scanning is carried out by a driving means such as a pulse motor, the driving is effected in an open loop manner, whereby the reliability of fringe scanning accuracy is not always sufficient, which is desired to be improved.